1. Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA
2. General Atomics, P.O. Box 85608, San Diego, California 92186-5608, USA
3. University of California–San Diego, 9500 Gilman Drive, La Jolla, California 92093, USA
4. Department of Engineering Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706-1687, USA